feat: Add CelebrityFinder new algorithm with Junit tests

Author: Hardvan

src/main/java/com/thealgorithms/stacks/CelebrityFinder.java

@@ -0,0 +1,40 @@
+package com.thealgorithms.stacks;
+
+/**
+ * Solves the celebrity problem using a stack-based algorithm.
+ *
+ * <p>Celebrity is someone known by everyone but doesn't know anyone else.
+ * <p>Applications: Graph theory and social network analysis.
+ *
+ * @author Hardvan
+ */
+public final class CelebrityFinder {
+    private CelebrityFinder() {
+    }
+
+    /**
+     * Finds the celebrity in the given party matrix.
+     *
+     * @param party A 2D matrix where party[i][j] is 1 if i knows j, otherwise 0.
+     * @return The index of the celebrity, or -1 if there is no celebrity.
+     */
+    public static int findCelebrity(int[][] party) {
+        int n = party.length;
+        int candidate = 0;
+
+        // Find a potential celebrity
+        for (int i = 1; i < n; i++) {
+            if (party[candidate][i] == 1) {
+                candidate = i;
+            }
+        }
+
+        // Verify the candidate
+        for (int i = 0; i < n; i++) {
+            if (i != candidate && (party[candidate][i] == 1 || party[i][candidate] == 0)) {
+                return -1;
+            }
+        }
+        return candidate;
+    }
+}

src/test/java/com/thealgorithms/stacks/CelebrityFinderTest.java

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+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class CelebrityFinderTest {
+
+    @Test
+    public void testCelebrityExists() {
+        int[][] party = {
+            {0, 1, 1},
+            {0, 0, 1},
+            {0, 0, 0}
+        };
+        assertEquals(2, CelebrityFinder.findCelebrity(party));
+    }
+
+    @Test
+    public void testNoCelebrity() {
+        int[][] party = {
+            {0, 1, 0},
+            {1, 0, 1},
+            {1, 1, 0}
+        };
+        assertEquals(-1, CelebrityFinder.findCelebrity(party));
+    }
+}